1. Field of the Invention
The invention relates in general to a vertical cavity surface emitting laser (VCSEL). More particularly, the invention relates to double-intracavity contacted long-wavelength vertical cavity lasers.
2. General Background and State of the Art
Semiconductor lasers are widely used in optical applications, in part because semiconductor fabrication techniques are relatively inexpensive and yield reliable, consistent results. Also, they are easily packaged into current microelectronics. A relatively new class of semiconductor lasers, vertical cavity surface emitting lasers (VCSELs), has been developed through the evolution of this technology. Unlike conventional edge emitting lasers that emit light in a direction parallel to the semiconductor substrates where the lasers are formed, VCSELs have optical cavities perpendicular to the substrate, and thus emit optical radiation in a direction perpendicular to the substrate and perpendicular to a p-n junction formed between layers grown on the substrate. In addition to various performance and application-adaptable improvements created thereby, VCSELs simply require reduced complexity in their fabrication and testing, as compared to conventional edge emitting semiconductor lasers.
Vertical cavity surface emitting lasers (VCSELs) have been proven to be solutions for low-cost transmitters for high-speed data communications at 980 nm and 850 nm and have shown great potential for cost-effective telecommunication systems at longer wavelengths as well, such as 1.55 xcexcm and 1.3 xcexcm. These long wavelength VCSELs will satisfy increasing demand for high speed data transmission over tens of kilometers. 10-Gigabit Ethernet is one example, which requires inexpensive transmitters with a data rate of 10 G bit per second (Gbps) and up to 40 km reach over single-mode fiber.
VCSELs are semiconductor lasers having a semiconductor layer of optically active material, such as gallium arsenide or indium gallium arsenide or the like, sandwiched between highly-reflective layers of metallic material, dielectric material, epitaxially-grown semiconductor dielectric material or combinations thereof, most frequently in stacks. As is conventional, one of the mirror stacks is partially reflective so as to pass a portion of the coherent light built up in the resonating cavity formed by the mirror stack/active layer sandwich. Laser structures require optical confinement and carrier confinement to achieve efficient conversion of pumping electrons to stimulated photons (a semiconductor may lase if it achieves population inversion in the energy bands of the active material.)
While both short wavelength and long wavelength VCSELs have proven to offer excellent solutions for many applications in the evolving optical applications marketplace, they also have certain limitations and drawbacks that are well known in the art. Some of the drawbacks are associated with the need to electrically pump as well as conduct heat through the multi-layer mirror stacks, which exhibit poor electrical and thermal conductivities. Other drawbacks to VCSEL performance include high threshold current and high operating voltage. Mirror stacks lattice-matched to InP, as desired for high-reliability long wavelength operation, have limited thermal conductivity, and if doped sufficiently to provide useful electrical conductivity, also create excessive optical loss.
The present invention provides a novel approach to overcoming the drawbacks of existing VCSELs by presenting a double intracavity contacted VCSEL having a selectively apertured active region. The present invention provides a new design and method of manufacture allowing for room temperature, continuous-wave (CW) operation of AsSb-based VCSELs by providing thick indium phosphide (InP) cladding layers having very thin heavily-doped contact layers in a double-intracavity structure.
The present invention has several features combined together resulting in improved thermal characteristics as well as reduced current and optical losses. These combined features include:
1. thick n-type cladding layers;
2. very thin, heavily-doped, n-typed contact layers and a very thin, heavily-doped, tunnel junction; and
4. an aperture formed from the active region.
The thick, n-type cladding layers provide low-impedance current and heat paths avoiding conduction through poor optically and electrically conducting distributed Bragg reflectors, also known as DBRs or Bragg mirrors. The low-impedance heat path reduces the device temperature. The low-impedance current path results in a lower device operating voltage which also reduces the device temperature. Because the current path is through the cladding layers, there is no need to dope the DBRs and substrate, resulting in reduced free carrier absorption and reduced optical loss. Optical loss is also decreased because the cladding layers are n-type rather than p-type.
The very thin, heavily-doped n-type contact layers lower the impedance of the current path by providing better electrical contact with attached contacts. The heavily-doped tunnel junction provides a hole source allowing both cladding layers to be substantially n-type, reducing optical loss and decreasing impedance. The heavily-doped contact layers are thin, reducing optical loss since the majority of the contact layer volume can be doped at a lower level. Additionally, the heavily-doped contact layers are located at standing-wave nulls in the laser cavity, reducing free carrier absorption. Selective etchings enable the exposure of such thin, heavily doped, contact and tunnel junction layers.
The aperture formed in the active region confines the current to the desired area of the active region, reducing the device voltage. The optical mode is also confined, reducing optical loss and lowering the threshold current. The optical mode is confined by the aperture, resulting in reduced optical loss at the sidewall of the etched-pillar DBR, lower threshold current, lower threshold current density, larger differential quantum efficiency and higher output power.
The method of increasing the processing efficiency of a VCSEL also includes removing a first, or top, DBR by applying a first selective etch, removing a first, or top, cladding layer by applying a second selective etch, and removing the active region by applying a third selective etch to form an undercut aperture in the active region into which electric current can be confined. Additionally, the present invention further provides the first room temperature, continuous-wave (CW) operation of a 1.55-xcexcm vertical-cavity surface-emitting laser (VCSEL) that is completely lattice-matched to InP and produced in one epitaxial growth.
Accordingly, it is one object of the present invention to provide for room temperature, continuous-wave (CW) operation of AsSb-based VCSELs by including indium phosphide (InP) cladding layers having very thin heavily-doped contact layers in a double-intracavity structure. It is another object of the invention to provide a method of increasing the processing efficiency of a VCSEL by selectively etching an undercut aperture into the active region and injecting current into the active region.
Another object of the present invention is to provide a room temperature, continuous-wave (CW) operation of a 1.55-xcexcm vertical-cavity surface-emitting laser (VCSEL) that is completely lattice-matched to InP and produced in one epitaxial growth.
Other objects of the present invention are to reduce elevated temperatures in the VCSEL, minimize optical loss associated with heavily-doped contact layers, reduce threshold current and operating voltage, and improve thermal conductivity and electrical resistivity in the VCSEL.